1. Field of the Invention
The present invention relates generally to a communication system, and more particularly, to a method of compensating a frequency error and an apparatus for the same in the communication system.
2. Background of the Related Art
Typically, a conventional frequency error compensation method in a communication system will be explained with reference to the accompanying drawings.
FIG. 1 is a block diagram of a conventional synchronous demodulating apparatus in a CDMA system.
Referring to FIG. 1, the conventional synchronous demodulating apparatus includes a despreader 10 for despreading a received signal r(t) with a Pseudo Noise (PN) code, a Walsh decover 20 for discovering the despread signal outputted from the despreader 10 with a predetermined Walsh code so as to obtain data symbols, a phase estimator 30 for estimating a phase error from a pilot signal of the despread signal outputted from the despreader 10, and a phase rotator 40 for compensating the received signal for the phase error estimated by the phase estimator 30.
Herein, the received signal r(t) is expressed by the following equation 1.r(t)=PN(t)(√{square root over (Pp)}+√{square root over (Pd)}w(t)d(t))ej(2×Δƒ(t)t+Δφ(t))  [Equation 1]
Wherein, PN(t) denotes a PSEUDO NOISE (PN) code, P_p a pilot power, P_d a data symbol power, w(t) a Walsh code, d(t) a data symbol, Δƒ(t) a frequency error, and Δφ(t) a phase error, respectively.
An output of the phase estimator 30 is given by the following equation 2.√{square root over (Pp)}ejΔφ(t)  [Equation 2]
An output of the Walsh decover 20 is given by the following equation 3.√{square root over (Pd)}d(t)ej(2×Δƒ(t)t+Δφ(t))  [Equation 3]
Finally, an output of the phase rotator 40 is given by the following equation 4.                                                         P              d                                ⁢                      d            ⁡                          (              t              )                                ⁢                      ⅇ                          j              ⁡                              (                                                      2                    *                    Δ                    ⁢                                                                                   ⁢                                          f                      ⁡                                              (                        t                        )                                                              ⁢                    t                                    +                                      Δ                    ⁢                                                                                   ⁢                                          ϕ                      ⁡                                              (                        t                        )                                                                                            )                                              ×                                    P              p                                ⁢                      ⅇ                          -                              jΔϕ                ⁡                                  (                  t                  )                                                                    =                                                            P                d                            ⁢                              P                p                                              ⁢                      d            ⁡                          (              t              )                                ⁢                      ⅇ                          j2πΔ              ⁢                                                           ⁢                              f                ⁡                                  (                  t                  )                                            ⁢              t                                                          [                  Equation          ⁢                                           ⁢          4                ]            
Herein, the phase estimator 30 comprises an accumulator 31 for accumulating the pilot signal, and a pilot filter 32 for estimating the phase error of the accumulated pilot signal.
At this time, components that distort the signal are produced due to phase and frequency differences, and especially, a frequency error may be produced due to a Doppler frequency shift caused by movement of a mobile station.
However, the conventional demodulating apparatus of FIG. 1 just detects the phase error through the phase estimator and corrects the phase error through the phase rotator, but does not correct the frequency error.
Thus, according to the conventional demodulating apparatus, though it does not matter in a static environment in which the size of the frequency error is small, the frequency error cannot be corrected in a mobile environment in which the mobile station moves, and this deteriorates a performance of receiving a signal.